US Secretary of State Hillary Clinton has insisted the department is moving swiftly and aggressively to strengthen security at US missions worldwide after the deadly September 11 raid on the Libya consulate.

In her last formal congressional testimony on Capitol Hill as America's top diplomat, Mrs Clinton once again took full responsibility for the department's mistakes leading up to assault at the US mission in Benghazi that killed Ambassador Chris Stevens and three other Americans.

Her voice cracking at one point, Mrs Clinton said the experience was highly personal.

"I stood next to President Obama as the Marines carried those flag-draped caskets off the plane at Andrews (Air Force Base). I put my arms around the mothers and fathers, sisters and brothers, sons and daughters," she told the Senate Foreign Relations Committee at a packed hearing.

Image Caption:Ambassador Chris Stevens

Her voice rising at another point, she fiercely defended UN Ambassador Susan Rice, who was vilified for widely debunked claims five days after the attack that protests precipitated the raid rather than terrorism.

"The fact is we had four dead Americans. Was it because of a protest? Or was it because of guys out for a walk one night who decided they'd go kill some Americans?

"It is our job to figure out what happened and do everything we can to prevent it from ever happening again, senator," she added.

Mrs Clinton said the department is implementing the 29 recommendations of an independent review board that harshly criticised the department as well as going above and beyond the proposals, with a special focus on high-threat posts.

The review board report faulted "systematic failures and leadership and management deficiencies at senior levels within two bureaus of the State Department" and four employees were put on administrative leave.

"Nobody is more committed to getting this right," she said. "I am determined to leave the State Department and our country safer, stronger, and more secure."

Three weeks after her release from a New York hospital, Mrs Clinton was at times defiant and willing to chastise lawmakers.

She will appear before the committee on Thursday to introduce her likely successor, Democratic Senator and former presidential candidate John Kerry.

Mrs Clinton refused to back down from withering Republican criticism of the Obama administration's shifting explanations about the assault.

She complained about the congressional holds placed on foreign aid and bilateral assistance. "We have to get our act together," she told the panel.

Her testimony focused not only on the attack but the growing threat from extremists in northern Africa, pointing out that Libya was not an isolated incident.

She said the Obama administration is pressing for a greater understanding of last week's hostage-taking and rescue effort in Algeria that left three Americans dead.

Clinton had been scheduled to testify before Congress last month, but an illness, a concussion and a blood clot near her brain forced her to postpone her appearance.

Politics play an outsized role in any appearance by Mrs Clinton, who sought the Democratic presidential nomination in 2008 and is the subject of constant speculation about a possible bid in 2016.

The former first lady and New York senator - a polarising figure dogged by controversy - is about to end her four-year tenure at the State Department with favourable ratings.

A poll early last month by the Pew Research Center for the People & the Press found 65% of Americans held a favourable impression of Mrs Clinton, compared with 29% unfavourable.